Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in an electrochemical cell is referred to as an electrode. Of a pair of electrodes used in an electrochemical cell, the electrode on the electrochemically higher potential side is referred to as the positive electrode or the cathode, while the electrode on the electrochemically lower potential side is referred to as the negative electrode, or the anode. A battery may contain one or more electrochemical cells.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. An electrochemical cell comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, an electrochemical cell comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
Discharging an electrochemical cell in its charged state by allowing electrons to flow from the anode to the cathode through an external circuit results in the electrochemical reduction of the cathode active material at the cathode and the electrochemical oxidation of the anode active material at the anode. To prevent the undesirable flow of the electrons in a short circuit internally from the anode to the cathode, an electrolyte element is interposed between the cathode and the anode. This electrolyte element must be electronically non-conductive to prevent short circuits, but must permit the transport of ions between the anode and the cathode. The electrolyte element should also be stable electrochemically and chemically toward both the anode and the cathode.
Typically, the electrolyte element contains a porous material, referred to as a separator (since it separates or insulates the anode and the cathode from each other), and an aqueous or non-aqueous electrolyte, that usually comprises an ionic electrolyte salt and ionically conductive material, in the pores of the separator. A variety of materials have been used for the porous layer or separator of the electrolyte element in electrochemical cells. These porous separator materials include polyolefins such as polyethylenes and polypropylenes, glass fiber filter papers, and ceramic materials. Usually these separator materials are supplied as porous free-standing membranes that are interleaved with the anodes and the cathodes in the fabrication of electrochemical cells. Alternatively, the porous separator may be applied directly to one of the electrodes, for example, as described in U.S. Pat. No. 5,194,341 to Bagley et al., and in U.S. Pat. No. 6,153,337 to Carlson et al.
Porous separator materials have been fabricated by a variety of processes including, for example, stretching combined with special heating and cooling of plastic films, extraction of a soluble plasticizer or filler from plastic films, and plasma oxidation. The methods for making existing free-standing separators typically involve the extrusion of melted polymeric materials either followed by a post-heating and stretching or drawing process or followed by a solvent extraction process to provide the porosity throughout the separator layer. U.S. Pat. No. 5,326,391 to Anderson et al., and references therein, describe the fabrication of free-standing porous materials based on extraction of a soluble plasticizer from pigmented plastic films. U.S. Pat. No. 5,418,091 to Gozdz et al., and references therein, describe forming electrolyte layers by extracting a soluble plasticizer from a fluorinated polymer matrix either as a coated component of a multilayer battery structure or as an individual separator film.
A liquid organic electrolyte containing organic solvents and lithium salts is typically used as the electrolyte in the pores of the separator in the electrolyte element for lithium-ion electrochemical cells. Alternatively, a gel or solid polymer electrolyte containing an ionically conductive polymer and lithium salts, and optionally organic solvents, might be utilized instead of the liquid organic electrolyte. For example, U.S. Pat. Nos. 5,597,659 and 5,691,005 to Morigaki et al. describe a separator matrix formed of a microporous polyolefin membrane impregnated in its pores with an ionic conductive gel electrolyte.
In addition to being porous and chemically stable to the other materials of the electrochemical cell, the separator should be flexible, thin, economical in cost, and have good mechanical strength. These properties are particularly important when the cell is spirally wound or is folded to increase the surface area of the electrodes and thereby improve the capacity and high rate capability of the cell. Typically, free-standing separators have been 20 microns or greater in thickness. As lithium-ion batteries have continued to evolve to higher volumetric capacities and smaller lightweight structures, there is a need for separators that are 15 microns or less in thickness. Reducing the thickness from 20 microns to 15 microns or less greatly increases the challenge of providing high porosity and good mechanical properties while not sacrificing the protection against short circuits or not significantly increasing the total cost of the separator in each battery.
High porosity in the separator is important for obtaining the high ionic conductivity needed for effective performance in most batteries, except, for example, those batteries operating at relatively low charge and discharge rates. It is desirable for the separator to have a porosity of at least 45 percent, and preferably 50 percent or higher, in lithium-ion batteries. As the separator is reduced in thickness from the typical 20 to 25 microns to 15 microns or less, the approximately 50 percent solids volume of the separator that is not voids or pores, must contribute all of the mechanical properties needed for fabrication into the electrochemical cell and for mechanical integrity during the storage and operation of the battery. Typically, lowering the porosity to increase the mechanical properties also reduces the ionic conductivity. This trade-off between high conductivity and good mechanical properties is a challenge in providing separators that are less than 25 microns in thickness, especially for those that are less than 15 microns thick.
The protection against short circuits is particularly critical in the case of secondary or rechargeable batteries with lithium as the anode active material. During the charging process of the battery, dendrites may form on the surface of the lithium anode and may grow with continued charging. A key feature of the separator in the electrolyte element of lithium-ion rechargeable batteries is that it has a small pore structure, such as 0.5 microns or less in pore diameter, and sufficient mechanical strength to prevent the lithium dendrites from contacting the cathode and causing a short circuit with perhaps a large increase in the temperature of the battery leading to an unsafe condition.
Another highly desirable feature of the separator in the electrolyte element is that it is readily wetted by the electrolyte materials that provide the ionic conductivity. When the separator material is a polyolefin material that has non-polar surface properties, the electrolyte materials (which typically have highly polar properties) often poorly wet the separator material. This results in longer times to fill the battery with electrolyte and potentially in low capacities in the battery due to a non-uniform distribution of electrolyte materials in the electrolyte element.
Further, it would be highly advantageous to be able to prepare separators by a relatively simple process of coating that directly provides ultrafine pores less than 50 nm in diameter and can readily provide a range of thicknesses from 40 microns or greater down to 1 micron.
A separator, particularly one with a thickness less than 15 microns, that is applicable for lithium-ion and other electrochemical cells, and that can reduce the trade-off between high ionic conductivity and good mechanical properties, would be of great value to the battery industry.